The present invention relates to an article comprising a substrate containing silicon and at least one layer and, more particularly, a layer which contains a coefficient of thermal expansion (CTE) tailoring additive in an amount sufficient to maintain a compatible CTE with at least one adjacent layer and the substrate.
Ceramic materials containing silicon and metal alloys containing silicon have been proposed for structures used in high temperature applications as, for example, gas turbine engines, heat exchangers, internal combustion engines, and the like. A particular useful application for these materials is for use in gas turbine engines which operate at high temperatures in aqueous environments. It has been found that these silicon containing substrates can recede and lose mass as a result of a formation volatile Si species, particularly Si(OH)x and SiO when exposed to high temperature, aqueous environments. For example, silicon carbide when exposed to a lean fuel environment of approximately 1 ATM pressure of water vapor at 1200xc2x0 C. will exhibit weight loss and recession at a rate of approximately 6 mils per 1000 hrs.
It is not uncommon to use one or more layers on a substrate such as barrier layers, bond layers, intermediate layers, and the like. It is important for the integrity of the final article to maintain a compatibility between the coefficient of thermal expansion (CTE) of the silicon containing substrate and adjacent layers and between adjacent layers themselves. If the CTE between adjacent layers and substrate are not compatible, the overall coating can break down at the interfaces between the non-compatible layers.
Accordingly, it is a principle object of the present invention to provide an article comprising the silicon containing substrate and at least one layer wherein the layer has a coefficient of thermal expansion which is compatible with the substrate when adjacent to the substrate and compatible with any other adjacent layer(s).
Further objects and advantages of the present invention will appear hereinbelow.
The CTE tailoring addition of the present invention may be used in any one or more layers (barrier layers, bond layers, intermediate layers, etc.) on a silicon containing substrate. The invention will be described with reference to barrier layers containing the CTE additive. In accordance therewith, an article comprises a silicon containing substrate having at least one layer, for example a barrier layer, wherein the barrier layer functions to both inhibit the formation of undesirable gaseous species of silicon when the article is exposed to a high temperature, aqueous environment and to provide thermal protection. By high temperatures is meant the temperature at which the Si in the substrate forms Si(OH)x and/or SiO in an aqueous environment. By aqueous environment is meant a water and/or steam environment. The silicon containing composite is preferably a ceramic or metal alloy containing silicon, The external barrier layer is characterized by a coefficient of thermal expansion which is within plus or minus 3.0 ppm per degree centigrade of the coefficient of thermal expansion (CTE) of the silicon containing substrate and/or any adjacent barrier layer. The CTE compatibility is accomplished by providing a CTE tailoring additive in the barrier layer in an amount sufficient to maintain the desired CTE compatibility. NbOx (where x=1 to 3 and is a ratio of O to the compound element, see U.S. Pat. No. 4,155,016) is, in accordance with the present invention, one suitable CTE tailoring additive. A further CTE tailoring additive in accordance with the present invention comprises NbOx and an oxide selected from the group consisting of MgO, CaO, SrO, BaO, Ta2O5, Al2O3, Y2O3, La2O3, Re2O3 (wherein Re is a rare earth element) and mixtures thereof, and this may also serve as the entire barrier layer if desired. The CTE tailoring additives described above may also be added to any known barrier layer or other layers used in combination on the substrate. Particularly useful barrier layers for use with the CTE tailoring additive include barrier layers selected from the group consisting of alkaline earth aluminosilicates, aluminum silicates, yttrium silicates, yttrium aluminosilicates, rare earth aluminosilicates, and oxides of aluminium, yttrium, magnesium, calcium, strontium, barium, tantalum, lanthanum, rare earth oxides, and mixtures thereof. The article can include one or more intermediate layers between the silicon based substrate and the barrier layer. The intermediate layer(s) serve(s) to provide enhanced adherence between the barrier layer and the substrate and/or to prevent reactions between the barrier layer and the substrate. In addition, bond layers and top layers may be included as is known in the art.